Where are you in your Black or White playthrough?

[Lady Vulpix]

I think the best place for Special Defense is the catacombs under the Desert Resort. The Volcarona hall is full of nothing but Claydols.

Oh, by the way, my Crobat has maxed out his EVs at level 77, making him the first Pokemon outside my main 12 to have max EVs.

Edit: my main team has reached level 100! All twelve of them! This is a first for me, since I'd never trained a Pokemon all the way to level 100 before. Now I can focus on training all the others I've been wanting to train, and my main 12 can help.

Edit 2: my second Ninetales and my Rotom maximized their EVs at level 52! And I have a big fat Snorlax (evolved at level 62)!